Pulmonary gas exchange depends heavily on diffusion which involves at least four gases: O2, CO2, N2 and water vapor. The phenomenon of multicomponent diffusion being more complicated than can be described by Fick's law of binary diffusion, the purpose of the proposed research is to investigate whether, and under what conditions the effect of multicomponent diffusion becomes significant enough to negate the use of Fick's law. This research consists of two stages, each containing a mathematical study and an experimental study. During the first stage detailed properties of ternary diffusion through porous membrane in the molecular diffusion region will be studied with both a mathematical model and an "open system" diffusion apparatus. Parametric studies will be made to determine the effects of total gas mixture pressure, gas mixture composition, and membrane thickness and porosity. At the second stage of the research the role of multicomponent diffusion in avian egg gas exchange will be assessed. Three mathematical models, representing varying degrees of simplification will be studied. Actual measurements of O2, CO2 tensions and water vapor loss will be made in incubating eggs. The effects of ambient pressure as well as different inert gases will be studied both mathematically and experimentally.